Clostridium botulinum, a spore-forming, heat-resistant, anaerobic bacterium, produces a protein-based toxin (botulinum toxin) having several serotypes, known as A through G serotypes, of which serotype C has 3 subtypes, known as serotypes C1, C2 and C3. The C1 neurotoxin paralyzes people and animals in low doses by blocking acetylcholine release by neurons, recovery is slow-treatment may require ventilation for multiple weeks before a person is able to breathe again. The C2 toxin is not a neuro-active, and causes necrosis and hemorrhaging. The C3 toxin is the least characterized of the C sub-serotypes.
Most toxin-based delivery systems are multi-domain proteins that bind target cells and translocate material (payloads) across the lipid bilayer into the cytosol of the targeted cell. These systems are altered AB-type toxins, consisting of a payload domain (A) and a binding/translocation domain (B). The A and B domains can be covalently linked by a polypeptide or disulfide bond that is later cleaved during the translocation step. Non-covalently linked (binary) A and B toxin domains are transcribed and translated independently and associate prior to exerting toxicity. The Clostridium botulinum C2 toxin (C2) is not a neurotoxin, but it has a binary AB toxin design.